In many applications, it is necessary to transmit a reference frequency to a number of stations over kilometer distances. The signals must have substantially the same phase stability and amplitude at all distribution points for use by signal processing equipment.
One precise frequency signal source is a hydrogen maser, but the maser output varies in amplitude. While an automatic gain control (AGC) circuit may be used at each station to maintain constant amplitude at each distribution point, an electrical AGC attenuator introduces phase shifts in the RF timing signal as a function of attenuation. The result would then be reference signals at the distribution points varying widely in phase so that phase stability of the various stations would be lost.
To avoid this phase shift in the attenuator, compensation circuits utilizing phase locked loops have been employed. Such resolution to the problem is unsatisfactory. The phase lock loops are complex and are subject to the usual component failure rates for electronic equipment. What is required is an improved and simplified distribution system with AGC at each distribution point and without any significant phase shift due to the reduction of amplitude changes.